Chicken liver dihydrofolate reductase is shown to contain a single cysteine residue and thus a single sulfhydryl group. This functional group is found to be readily titrated by both p-hydroxymercuribenzoate and dithionitrobenzoate although the dithionitrobenzoate reacts much more slowly than the organic mercurial. The binding of p-hydroxymercuribenzoate as well as a series of other organic mercurials such as methylmercuric hydroxide results in a marked stimulatiion of the reductase reaction. In contrast, the binding of thionitrobenzolate results in inhibition of activity. Evidence is presented that suggests that this sulfhydryl group is not at the active site of the enzyme since the interaction with the mercurials is not affected by the presence of a large excess of substrate. The results suggest that binding of a specific substance to this sulfhydryl group results in a conformational change in the protein yielding a more active enzymatic species. However attempts to demonstrate such a conformational change by a variety of methods have not been successful. Thus the effect on the tertiary structure of the protein must be very small. Kinetic studies suggest that the end result of this change may be related to the binding of the substrates. BIBLIOGRAPHIC REFERENCE: Kaufman, B.T. and Kemerer, V.F.: Characterization of Chicken Liver Dihydrofolate Reductase after Purification by Affinity Chromatrography and Isoelectric Focusing. Arch. Biochem. Biophys. 179, 420, 1977.